1. Field of the Invention
The present invention relates to a printed wiring board-forming sheet, a via hole-forming method, a process for producing a resin sheet having a filled via hole, a device for forming the via hole, a TAB (tape automated bonding) tape, CSP (chip size package), BGA (ball grid array), FPC (flexible printed circuit), a so-called multi-layer board using a rigid substrate such as glass epoxy, a multi-layer printed wiring board capable of being produced by the use of a punching press, and a process for producing the same.
2. Description of Related Art
A conventional process for producing a resin sheet having a filled via hole is described below with reference to FIG. 25.
In the first place, a die having an upper part 112 provided with a punch 110 and a lower part 116 (sometimes referred to as a “base”) having a die hole 114 formed at the position corresponding to the punch 110 is used, and between the upper part 112 and the lower part 116, a resin sheet 118 (thin resin sheet such as polyimide resin sheet or glass epoxy resin sheet) to be provided with a through hole is placed (FIG. 25(a)).
In the next place, the upper part 112 is lowered to allow the punch 110 to penetrate the resin sheet 118, whereby a punched hole 120 is formed in the resin sheet (FIG. 25(b)).
In the last place, a conductor paste 122 is squeezed into the punched hole 120 of the resin sheet 118 by a screen printing method using a metal mask (not shown) having an opening corresponding to the punched hole 120 and a squeegee (not shown). Thus, a via hole 124 filled with a conductor can be formed in the resin sheet 118.
Next, a process for producing a semiconductor device using the resin sheet 118 is described.
In the first place, both surfaces of the resin sheet 118 having a via hole 124 are subjected to electroless copper plating and then electrolytic copper plating to form conductor layers 126, as shown in FIG. 26(a). It is possible to form a conductive layer by means of adhesion bonding of a copper foil.
In the next place, the conductor layers 126 are patterned by photolithography, whereby a first pad 128 to be brought into contact with an electrode terminal of a semiconductor chip and a first wiring pattern 130 for connecting the first pad 128 to the upper end of the via hole 124 are formed on the upper surface of the resin sheet 118. Likewise, a second pad 132 for mounting thereon an external connecting terminal (e.g., solder ball) and a second wiring pattern 134 for connecting the second pad 132 to the lower end of the via hole 124 are formed on the lower surface of the resin sheet (FIG. 26(b)).
In the last place, a semiconductor chip 136 is mounted on the upper surface (surface for mounting semiconductor chip) of the resin sheet 118 to electrically connect an electrode pad 138 of the semiconductor chip 136 to the first pad 128, and an external connecting terminal 140 to be mounted on a mounting substrate (not shown) is set on the second pad 132 formed on the lower surface of the resin sheet 118. The external connecting terminal is, for example, a bump. A pin is also available.
Through the above process, a semiconductor device 142 using the resin sheet 118 is completed (FIG. 26(c)).
Plural resin sheets 118 each having a via hole 124 may be laminated to form a multi-layer board.
Using the insulating film (insulating substrate), a printed wiring board having wiring patterns formed on both surfaces of the insulating substrate can be produced, as described above. Examples of the printed wiring boards having wiring patterns on both surfaces include a TAB (tape automated bonding) tape, CSP (chip size package), BGA (ball grid array) and FPC (Flexible printed circuit), each of which uses a flexible substrate such as polyimide. A so-called multi-layer board using a rigid substrate such as glass epoxy is also available.
For producing the printed wiring board having conductor layers on its front and back surfaces, it is necessary to previously form a hole such as a through hole or a blind via hole at the desired position by means of a punching press, a drill or a laser beam. When a laser beam is used, it is necessary to remove so-called smears produced by a heat of the laser beam (desmearing operation).
In a process wherein the hole opened as above is filled with a conductive paste or the like using a screen printing machine to electrically connect the conductor layers on the front and back surfaces (referred to as a “conductive paste printing process” hereinafter), time-consuming steps including adhesion of a backing sheet after a hole opening in order to prevent passing of the paste into the backside, printing of the conductive paste, curing thereof and peeling of the backing sheet are required in case of the through hole. In case of the blind via hole, the diameter of an opening of a mask should be accurately determined in order to completely fill the hole, and therefore the printing machine needs to be equipped with an image recognition device or the like having high positional accuracy. On this account, the printing machine becomes expensive, and as a result, the product cost is increased.
In another process, the front and back surfaces of the insulating substrate can be electrically connected by plating on the circumferential wall of the via hole. In this case, however, it is necessary to preform electroless copper plating on the through hole or application of carbon or the like, followed by electrolytic copper plating. (This process is referred to as a “plating process” hereinafter). This process is a wet process, so that there resides a problem in disposal of waste liquid.
In the field of this art, it has been desired to improve the above processes in the cost, setting aside the reliability about the electrical connection.
For example, proposals to solve the problem have been made in Japanese Patent Laid-Open Publications No. 61992/1984 and No. 81789/1987.
In Japanese Patent Laid-Open Publication No. 61992/1984, there is disclosed a process for producing a through hole printed wiring board, which comprises forming wiring patterns, each having a conductive part covering a through hole on both surfaces of an insulating substrate, punching a hole having a diameter smaller than that of the through hole at the center of the conductive part, bringing the conductive parts on both surfaces into contact with each other and fixing the conductive parts with conductive members to electrically connect the copper foils on both surfaces to each other.
This process, however, is complicated because it includes steps of formation of a through hole, punching of a small hole and fixation with conductive members. Further, the surface smoothness becomes poor because the conductive part on one side is brought into contact with the conductive part on the other side by virtue of sagging, and besides, this process lacks reliability about the electrical connection.
In Japanese Patent Laid-Open Publication No. 81789/1987, there is disclosed a process for producing a wiring board, which comprises punching a through hole into an uncalcined ceramic substrate (green sheet), then inserting a connecting pin in the through hole, printing a conductor layer on a surface of the substrate by screen printing, and calcining the green sheet.
This process uses a green sheet as a substrate as described above, and does not use an insulating substrate such as a polyimide substrate.
By the way, a multi-layer board obtained by laminating plural printed wiring boards, each of which has a wiring pattern formed on its front and/or back surface has been used.
In Japanese Patent Laid-Open Publication No. 125344/1996, a build-up process using such a printed wiring board is disclosed. This process is described below with reference to FIGS. 27(a) to 27(d).
In the first place, an insulating substrate 401 having a first copper layer formed on one surface is subjected to masking and etching to form a desired first wiring pattern 402, and a conical first conductive bump 403 is printed on the wiring pattern 402. Above the insulating substrate 401, a first insulating adhesive layer 404 having the same shape as the insulating substrate and having a second copper layer 402a formed on its upper surface is located (FIG. 27(a)). Then, the insulating adhesive layer 404 is moved and press-bonded to the insulating substrate 401. As a result, the tip of the conical first conductive bump 403 is not only collapsed to be flattened but also allowed to penetrate the insulating adhesive layer 404 and come into contact with the second copper layer 402a (FIG. 27(b)). Thus, a first laminate 405 is produced.
In the next place, the surface of the second copper layer 402a of the first laminate 405 shown in FIG. 27b is subjected to masking and etching to form a desired second wiring pattern 402b, and on the wiring pattern 402b, a second conductive bump 403a having the same shape as the first conductive bump is printed. Then, a second insulating adhesive layer 404a having a third copper layer 402c formed on its upper surface is located above the first laminate 405 (FIG. 27(c)).
The second insulating layer 404a is moved downward and press-bonded to the first insulating adhesive layer 404. As a result, the tip of the conical second conductive bump 403a is not only collapsed to be flattened but also allowed to penetrate the second insulating adhesive layer 404a and come into contact with the third copper layer 402c. The third copper layer 402c is subjected to masking and etching to form a desired third wiring pattern 402d. Thus, a second laminate 405a is produced (FIG. 27(d)).
In the build-up process shown in FIG. 27 that is a conventional process, in order to form plural wiring patterns 402, 402b and 402d on the insulating substrate 401 through the insulating adhesive layers 404 and 404a, the same number of operations of printing the conductive bumps and press-bonding the insulating adhesive layers as that of the wiring patterns must be carried out. In other words, formation of a wiring pattern and printing of a conductive bump are necessary for each layer.
Moreover, with a demand for fine printed wiring boards, the wiring pattern to be formed becomes finer and finer, so that it is not easy to form a bump corresponding to the fine wiring pattern by a printing method. In addition, it is extremely burdensome to conduct the same number of printing operations as that of the wiring patterns, and in the production of a large number of products, time loss and economical loss are not negligible.
To fill the demand for fine wiring pattern printing, a printing machine equipped with an image recognition device, which has not only high printing performance but also excellent image recognition function and high positional accuracy, is necessary. Such a printing machine, however, is generally expensive, and the equipment investment becomes enormous.
In the conventional technique shown in FIG. 27, a wiring pattern layer can be formed on the lower side of the insulating substrate 401 in a manner similar to the above, but in this case, a through hole that passes through the insulating substrate 401 must be formed in order to electrically connect the wiring pattern on the upper side of the insulating substrate 401 to the wiring pattern on the lower side thereof, and this requires not only formation of the through hole but also plating on the through hole. That is, the process becomes extremely complicated.
It is an object of the present invention to provide a method for forming a via hole capable of surely attaining electrical connection between the front and back surfaces of a resin sheet.
It is another object of the present invention to provide a printed wiring board-forming sheet capable of surely attaining electrical connection between the front and back surfaces of a resin sheet.
It is a further object of the present invention to provide a device for forming the above-mentioned via hole or a filled via hole.
It is a still further object of the present invention to provide a process for producing a resin sheet having a filled via hole, in which the production steps are simplified and the filled via hole can be surely filled with a conductor.
It is a still further object of the present invention to provide a printed wiring board having high reliability about the electrical connection between the front and back surface conductive layers and capable of being produced at a low cost, and to provide a process for producing the printed wiring board.
It is a still further object of the present invention to provide a process for producing a multi-layer printed wiring board, which comprises laminating plural printed wiring boards each having wiring patterns formed on the front and back surfaces to ensure favorable electrical connection in the thickness direction, and to provide a multi-layer printed wiring board.